Abstract In vitro models of tissue barriers such as the gut, lung, and vasculature are important for understanding the basis of disease and for assessing the ability of drug formulations to reach target tissues. Despite the growth of sophisticated 3D (also microphysiological and tissue chip) culture systems, the simplest and most popular tools for the in vitro study of barrier tissues remains the Corning Transwell? and its competitors (collectively referred to here as ?Transwells??). These products suspend a thick (~ 10 m ) polymer membrane in a culture well to create apical and basal compartments separated by a monolayer or co-culture grown on the membrane. Despite their popularity, Transwells? are notoriously bad for cell imaging and do not provide the fluid flow needed to properly condition vascular barriers and to study immune cell trafficking. Here, we propose to use SiMPore's ultrathin (< 300 nm), highly permeable, and optically transparent membranes to create a cell culture platform that overcomes these limitations. Our project will create a modular platform featuring a core unit called the SiM (microphysiological system enabled by a Silicon Membrane; developed in Aim 1) that readily converts into a flow cell through the addition of a ?plug-and-play? flow module (developed in Aim 2). The SiM will enable live cell and high resolution microscopy in an open-well format that is familiar to Transwells? users. To suit different applications, the SiM will feature one of three SiMPore membranes: 1) nanoporous, 2) dual nano and microporous; and 3) 0.5 m pores. The SiM will convert into a flow cell through the addition of a flow module that aligns and seals via magnetic latches. In this way users can initiate culture in an easy-to-use open-well device before initiating flow. Non-modular open well and flow cell devices featuring SiMPore membranes are already produced for academic collaborators by PI McGrath (University of Rochester) in a one-by-one manner that cannot support broader distribution. This STTR will create a commercial alternative using high-throughput manufacturing to achieve unit costs in line with those of Transwells?. In addition to high yields in manufacturing (> 90%), our success metrics will ensure the SiM platform achieves the basic functionality of the lab-crafted devices. Specifically we will verify: 1) endothelial barrier maturation as indicated by tight junction formation and low permeability to small molecule diffusion (<1.7 x 10-6 cm/sec for 4kDa FITC dextran); 2) barrier enhancement and endothelial alignment in response to the application of physiological levels of shear (10 dynes/cm2); and 3) the ability to introduce leukocytes under flow and witness each stage of trafficking across a vascular barrier (rolling, luminal and abluminal crawling, diapedesis). Results will be externally verified through the McGrath lab collaborations. Phase I will produce both the SiM and the SiM flow module as products. Phase II will introduce additional functional modules (TEER, ELISA) and multiplex formats.